Measurement While Drilling (MWD) and Logging While Drilling (LWD) systems derive much of their value from the ability to provide real-time information about conditions near the drill bit. Oil companies use these downhole measurements to make decisions during the drilling process, and sophisticated drilling techniques, such as the GeoSteering system developed by Schlumberger, Ltd. Such techniques rely heavily on instantaneous knowledge of the formation that is being drilled. The industry continues to develop new measurements for MWD/LWD, including imaging type measurements with high data content.
These new measurement and control systems require telemetry systems having higher data rates than those currently available. As a result, a number of telemetry techniques for use with measurement while drilling have been tried or proposed.
The industry standard is mud pulse telemetry that uses the drill pipe to guide acoustic waves in the drilling fluid. Currently, using mud pulse telemetry, data is sent to the surface at bit rates in the range of 1-6 bits/second. Such a slow rate is incapable of transmitting the large amounts of data that are typically gathered with an LWD string. In some cases (e.g., foamed drilling fluid), mud pulse telemetry does not work at all. Normally, some or all of the data is stored in downhole memory and downloaded at the end of the bit run. This delay significantly reduces the value of the data for real-time applications. Also, there is a significant risk of data loss, for example, if the tool is lost in the hole.
Electromagnetic telemetry via earth path has been tried with limited success. Even at very low data rates, it works only to a limited depth, depending on the resistivity of the earth.
Acoustic telemetry through the drill pipe itself has been studied extensively but not used commercially, so far. In theory, data rates in the 10's of bits/second should be possible using acoustic waves in the steel.
The idea of putting a wire in the drill pipe has been proposed numerous times over the past 25 years. Shell and Exxon each reportedly built an experimental wired drill string in the late 1970's. Prior art relating to these efforts is disclosed in U.S. Pat. No. 4,126,848 to Denison, “Drill String Telemeter System”; U.S. Pat. No. 3,957,118 to Barry et al., “Cable System for use in a Pipe String and Method for Installing and Using the same”; and U.S. Pat. No. 3,807,502 to Heilhecker et al., “Method for Installing an Electric Conductor in a Drill String”; and the publication “Four Different Systems Used for MWD”, W. J. McDonald, The Oil and Gas Journal, pp 115-124 , 3 Apr. 1978. Such systems are believed to have suffered from poor reliability and high cost because of the large number of electrical connectors.
IFP developed a system known as “Simphor” which used wireline cables and large, robust wet connectors. It has never been commercialized for measurement while drilling applications. This system is believed to have suffered from interference with the drilling process.
The use of inductive couplers in drill pipe is known. U.S. Pat. No. 4,605,268, to Meador, “Transformer cable connector” describes the use and basic operation of inductive couplers mounted at the sealing faces of drill pipes. Russian patent 2,140,537, “A system for transmitting electrical energy and data within a column of adjoining tubes”, November 1999, describes a drill pipe telemetry system that uses inductive couplers. WO Publication 90/14497A2, by Eastman Christensen GMBH, “Process and device for transmitting data signals and/or control signals in a pipe train” describes an inductive coupler mounted at the ID of the drill pipe joint for data transfer.
Other U.S. patents are as follows: U.S. Pat. No. 5,052,941 to Hernandez-Marti et al., “Inductive coupling connector for a well head equipment”; U.S. Pat. No. 4,806,928 to Veneruso, “Apparatus for electro-magnetically coupling power and data signals between well bore apparatus and the surface”; U.S. Pat. No. 4,901,069 to Veneruso, “Apparatus for electro-magnetically coupling power and data signals between a first unit and a second unit and in particular between well bore apparatus and the surface”; U.S. Pat. No. 5,521,592 to Veneruso; “Method and apparatus for transmitting information relating to the operation of a downhole electrical device”; U.S. Pat. No. 5,278,550 to Rhein-Knudsen, et al., “Apparatus and method for retrieving and/or communicating with downhole equipment”; and U.S. Pat. No. 5,971,072 to Huber et al., “Inductive coupler activated completion system”.
None of these references has provided a telemetry system for reliably transmitting measurement data at high data rates from locations near the drill bit to a surface station. Therefore, there exists a need for a telemetry system for reliably transmitting measurement data at high data rates to a surface station from locations in a borehole.